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Wind gustiness is one of the risk factors to be considered in the planning of landing and take-off operations. It is important to understand which runway directions are subjected to the worst risk of strong along- and cross-track gusts, and the conditions under which the greatest risks occur. This is of use both in streamlining air traffic operations under increasing traffic densities, and in planning future airport expansions.
Models of wind gustiness used by the structural engineering community have been
rigorously developed over the last few decades. Ultimately, mathematical
models of the wind behaviour such as that developed by Deaves and
Harris
form the basis of official standards used by engineers in the design of large
buildings and structures. These models are invariably based on probabilistic
techniques, the aim being to have sufficient well-modelled data to formulate
design criteria. Models of gustiness continue to be a subject of research
concern for wind engineers
.
In contrast, aircraft operations are not governed by rigid mathematically-based
criteria. Specific wind gust tolerances are available for particular models of
aircraft, but for aircraft in general there are few quantitative criteria for
operations in gusty weather. Specific flight strategies have now been developed
for the deterministic flow generated by a microburst
However, for generally gusty weather not necessarily created by a known wind
field like a microburst, aircraft safety must still be assured through risk
assessment rather than by relying on avoidance strategies. Ultimately,
decisions affecting the safety of an individual aircraft are always the
prerogative of the pilot in command, who must make a decision based on the
broadcast terminal information,
experience and a feel for the conditions of the moment. A landing aircraft
normally makes its final approach at no less than 
times its stalling speed. In practice, pilots always seek to land at this
minimum safe speed, since landing faster than absolutely necessary can lead to
other risks. This rule can be used to gauge the significance of particular
gust regimes to landing aircraft. For example, in the extreme case of a gust
reducing an aircraft's final-approach speed by 23%, it would stall, with
disastrous consequences.
The subject of this paper is therefore the surface wind gust regime and its influence on aircraft safety. The analyses herein are essentially based in the time domain, using probabilistic techniques. This paper does not address wind shear, where there is a sustained difference in wind velocity across an aircraft's path. This requires an analysis in the spatial domain and is to be the subject of a future paper.
R. Manasseh Papers
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